The present invention relates to an improved method allowing the incorporation of a metallic structure into a plastic with a view to producing molded articles; it also relates to a novel type of metallic structure particularly suited for the implementation of such a method as well as the molded articles including such a structure, and which have the characteristic of reflecting or barring electromagnetic waves and/or of being a good conductor of static electricity.
In the rest of the description, the term "plastic" will be used in the wide sense so as to designate any type of material such as, for example:
thermoplastic materials like polyolefins (polyethylene, polypropylene), styrene-based materials (ABS, polystyrene, ASA, SAN), polyamides, thermoplastic polyesters (PET, PBT), polycarbonates, PMMAs (polymethyl methacrylate), PVC, etc., as well as the mixtures of these various materials (blends) PA1 thermosettable materials, like unsaturated polyesters, epoxies, polyurethanes, phenolics, etc., PA1 elastomers like natural rubber, synthetic rubbers (SBR, nitrile rubber), thermoplastic elastomers (styrene-based, polyolefin-based, polyurethane-based or other thermoplastic elastomers) etc., which can be shaped by any type of process, such as, for example, injection molding, thermoforming, extrusion blow-molding, rotomolding, or other processes. PA1 the shrinkage which leads to irregularity in the thermoplastic yarn meshwork, PA1 a "spiralled" deformation which leads to poor reflection of the waves and lower conductivity because of lack of contact. PA1 a copper line or strip having a width lying between 0.3 mm and 1.5 mm and a thickness of from 0.001 mm to 0.003 mm will preferably be used as the metal yarn, this yarn preferably being silver plated in order to obtain better conductivity and reflection of the waves; PA1 the metal line or strip is preferably combined with a synthetic yarn which bonds easily to the thermoplastic material with which it will form a composite, using approximately 5 to 10% by weight of the metal yarn; in such a case, although the use of textile strips or lines presheathed with plastics can be envisaged, it is advantageous to produce such a combination on the same hitting machine, by hitting the two filamentary elements (synthetic yarn and metal), in a plated pattern, that is to say making it possible to have superposition of two elementary yarns in the knitted fabric formed, and this is done in such a way that one of the faces of said knitted fabric consists for the most part of yarns of one type (metal), whereas the other face, for its part, consists for the most part of synthetic yarns; the synthetic yarn can be of the same type as the plastic, ensuring good bonding to the component of the thermoplastic sheet; moreover, it is possible to use an extensible yarn, such as, for example, a textured yarn or a highly elastic yarn, such as those marketed under the LYCRA trademark or similar products, which makes it possible to cause the meshwork to shrink and leads to a denser knitted fabric upon leaving the machine; PA1 a knitting pattern will be used which is either a pattern of the plain-hitting type or, preferably, a pattern enabling weft float-loops to be obtained by means of tuck loops, so as to have a highly deformable structure.
It is well known that, during the production by molding of articles based on plastics, it is possible to incorporate therein various additives and/or reinforcements, and this is done in order to confer on them specific properties.
Thus, in order to confer on them properties ensuring electromagnetic (EMI) or electrostatic (ESD) protection, it was proposed a long time ago to incorporate into such plastics various fillers, such as metal powders. However, such a procedure has very many drawbacks, among which it is possible to cite the difficulties of having a very homogeneous composition.
One solution to solve this problem consists in covering the molded part with a reflective surface layer, which solution is especially used for producing reflective assemblies (antennas, for example). However, such a surface coating has the drawback of being subjected to external attack, and therefore may deteriorate over time. Moreover, such techniques require for their implementation complex and expensive equipment and processes.
In order to overcome these drawbacks, it was proposed, and this emerges especially from U.S. Pat. No. 4,242,686, to incorporate into the molded article, especially during its production, an intermeshed metallic structure. Although this solution enables shaped articles to be obtained in which the conducting or reflective element is perfectly protected from external attack, it does not, however, produce assemblies of complex shape and/or of small thickness taking into account the fact that the metallic structure is in the shape of a tubular sleeve which lacks pliability, and which, during its implementation, is flattened when it is impregnated with the plastic resin. Moreover, it clearly emerges from this document that the incorporation can only be envisaged solely for the production of articles of the "laminated" type, in which the material is produced by successive layers and not for being implemented according to similar techniques of composite forming or overmolding.
It has also been proposed, in EP-A-0,243,193, to produce conductive sheets or films, optionally shaped by making use of a textile structure (woven or knitted fabric), constituted by a twisted composite yarn which includes conductive fibers and thermoplastic fibers which have a hot shrinkage greater than that of the said conductive fibers. Such a yarn has the characteristic that, during a heat treatment, a shrinkage of the thermoplastic fibers occurs in the axial direction, which makes the conductive fibers become spiralled around the said thermoplastic fibers.
However, such a solution has drawbacks, amongst which it is possible to cite:
In many applications (for example, for the production of molded protective caps for computer equipment), metal, especially stainless steel, assemblies are still used which have the drawbacks of being complicated to use, time-consuming to install, and have a high cost.